This invention relates in general to tube-type vessels and more particularly to such a vessel, the tubes of which are joined to the tubesheet in a unique manner, and to the process for attaching the tubes to the tubesheet.
In tube-type heat exchangers and other vessels used for industrial processes, the tubes extend between tubesheets and have their ends secured to the tubesheets. One fluid fills the cavity between the tubesheets and surrounds the outsides of the tubes, while another flows through the tubes, so that heat is transferred through the tube walls from the hotter fluid to the cooler fluid. Various procedures and equipment have been developed for fastening the tubes to the tubesheets.
In the typical heat exchanger (FIG. 1a) each tube extends through a separate hole in the tubesheet and has its end edge flush with front face of the tubesheet. Here, the two are welded together so that the weld exists along the end edge of the tube. Since the weld is at the front face of the tubesheet, a small annular crevice exists between the outer surface of the tube wall and the surrounding surface of the tubesheet hole into which the tube fits. In some applications, particularly where the fluid that circulates amongst the tubes is a liquid, the fluid enters the crevices surrounding the tubes and transforms into a solid deposit that builds up and contracts the tube wall at the crevice. In some instances the girdling effect is sufficiently great to completely collapse some of the tubes (FIG. 1a--left).
One way to eliminate crevice build up, and crevice corrosion as well, is to weld the tubes to the back face of the tubesheet so that no crevices exist at the joints between the tubes and tubesheets. This may be achieved by welding around the periphery of each tube from the exterior of the tube, but welding in the confined areas amongst numerous closely spaced tubes is not an easy procedure. Moreover, it requires specially bent electrodes that are configured to reach around the tubes. Also, the tubes must be positioned vertically so that they can be welded in a down hand position. This requires a great deal of head room--often more than is available in some shops.
Another possibility is to weld the tubes to the tubesheet from within the holes of the tubesheet. In this regard, welding heads are available that reach deep within small holes to produce circular welds which completely penetrate the tube walls. However, one cannot observe the welding electrode as it orbits within the hole.
If the outside diameter of the tube is the same as or slightly smaller than the diameter of the hole within the tubesheet (FIG. 1b), it is difficult to position the tube accurately with respect to the electrode of the welding head, and as a result the welds which are produced are often deficient. For example, the tube may locate so deeply within the hole that the resulting weld does not achieve sufficient penetration to eliminate the crevice (FIG. 1b--left). On the other hand, the tube may not extend far enough into the hole, in which case burn-through might well occur (FIG. 1b--right). In short, the problem resides in the inability to precisely locate the tube in the hole.
The tube location problem may be overcome by making the holes in the tubesheets equal to the inside diameter of the tubes and providing the tubesheets at the end of the holes with sockets of predetermined depth for receiving the ends of the tubes (FIG. 1c). The tubes when fitted fully into the sockets are positioned precisely for welding from within the holes. While by this method it is possible to produce a fillet between the exterior surface of the tube and the backface of the tubesheet, the fillet takes metal away from the weld, leaving a depression and a cross-section that is thinner than desirable (FIG. 1c--left). The depression creates a weak point in the structure and may produce flow or other problems in some industrial processes. Also, a good possibility exists that at some locations around the joint, incomplete fusion will occur (FIG. 1c--right).